January 4, 2013

For Bering Sea Predators, Prey Density More Important Than Total Numbers

As ocean predators comb the waters in search of a food source, they often seek out dense groups of prey to maximize their hunting efforts, according to a new study in the open access journal PLOS ONE.

A team of American and Canadian biologists began their study by trying to find out where three different Bering Sea predators — northern fur seals and two different sea birds, black-legged kittiwakes and thick-billed murres — spent their time and energy pursuing prey.

"We had to think very differently about these interactions, trying to see the world from the predators' point of view," said Kelly Benoit-Bird, an Oregon State University marine ecologist and lead author on the study. "When we first tried to identify good foraging locations for predator species we looked at areas of high prey numbers because it makes sense that they'd be where the food is. But the results didn't match what we might have expected.”

The team found that the overall amount of prey — in this case krill or Pollock — in a region doesn´t matter as much to the well-being of a predator population as opposed to if these prey densely collected into attractive groups.

"Predator populations that should have been doing well, based on prey numbers or biomass, were in fact not doing well," added Benoit-Bird. "What we discovered is that smaller aggregations of prey are more attractive to predators if they are sufficiently dense."

"In a way, they're looking for the same thing that commercial fishing fleets look for — high-quality prey in aggregations dense enough to be economical," said study co-author Scott Heppell, an assistant professor of fisheries and wildlife at OSU.

In their study, the researchers tried to find a spatial relationship among these predators based on prey biomass and numerical abundance, they were unable to. However, when they located small patches of prey at certain depths and density, a correlation with predator activity became apparent.

"To be honest, we aren't really sure how these predators — which may travel many miles — locate the densest aggregations at depths well below the surface — and often at night," Heppell explained. “You wouldn't think murres and fur seals would have that much in common, but in this case they do."

The scientists also noticed another unique behavior in adult murres and kittiwakes, which capture nearby prey to feed their young during the day, yet make long treks at night to feast on deep clusters of prey before returning to their nests to feed their chicks.

"It is a trade-off strategy," said Benoit-Bird. "They feed themselves in one place and nourish their offspring from another."

A problem with this strategy is that prey aggregation seems to occur randomly. For example, pollock fish were only observed aggregating when a close-knit and growing group of individuals reached a certain threshold.

"If the population is sufficiently diffuse, the pollock don't aggregate and that could spell trouble for species that prey upon them," Heppell said. "A 10 percent shift in the number of fish could change how the entire stock behaves — and have a major impact on the birds, seals and other predators."